Comparison of sludge characteristics between lime and limestone/lime treatment of acid mine drainage

Abstract

The U.S. Geological Survey, in cooperation with the Colorado School of Mines and the U.S. Environmental Protection Agency, has demonstrated the application of pulsed limestone bed (PLB) treatment of acid mine drainage (AMD) at the Argo tunnel discharge near Idaho Springs, Colorado. Current technology for AMD treatment at the Argo facility is neutralization with lime. However, lime neutralization often results in large amounts of highly hydrated metal oxide sludge, leading to high disposal costs. Use of the PLB process as a pretreatment typically offers cost savings not only through lower reagent costs, but also through decreased sludge volume. In this study we compared the characteristics of sludges created using lime only to a sludge that was pretreated with the PLB, followed by lime treatment. Lime treatment was performed batchwise in a 60 gallon cone-bottom stirred tank where the pH was elevated to 10, and held for an hour. A sample of sludge from the operating treatment plant was also tested for comparison. The PLB/lime treatment was accomplished by first passing the water through the PLB system, followed by batch lime treatment to pH 10 as before. The sludge qualities were evaluated through settled sludge volume, filterability through a bench-scale filter press, and the moisture percentage in the filter cake. The PLB/lime treatment resulted in a decrease in sludge volume of 34% after 24 hours, with good filtration performance, and a final cake solids content of 22% versus 19% for the lime sludge. A decrease in lime consumption of 45% was also obtained with the PLB pretreatment.